Tunable lasers are lasers in which the frequency or color of the lasing light can be controllably altered. Tunable lasers have utility in a number of diverse applications, including but not limited to telecommunications, open air remote sensing for environmental monitors, distributed fiber sensors, holographic data storage, spectroscopy, atomic frequency and time standards, optical coherence tomography for medical imaging, laser cooling, lidar, and many more. As a specific example, the ability to control the wavelength of the laser light has enabled wavelength division multiplexing in telecommunications, thereby increasing the bandwidth of optical fibers. As a second specific example, common embodiments of distributed fiber sensors require a tunable laser interrogator, through which temperature, pressure, chemical analysis, or other measurable quantities can be probed in diverse environments such as along power lines, embedded inside oil wells, along bridges and tunnels, and many more. In a third example, tunable laser light can be passed through suspect regions of air and then the intensity measured. Wavelength specific absorption features can signify the presence of certain chemicals, contaminants, or other pollutants. As another specific example, in the emerging market of holographic data storage, tunable lasers can be utilized to compensate for temperature changes in the storage medium.
There are various types of conventional tunable lasers. Prominent examples include distributed Bragg reflector (DBR) lasers, distributed feedback (DFB) lasers, and external cavity diode lasers (ECDL). Multi-section DBR lasers can provide significant wavelength tuning (up to 100 nm), but not in a continuous fashion. Specifically, after the DFB laser tunes a small amount, the frequency may jump (mode-hop) in an often-uncontrollable way. DBR lasers can typically only change the laser wavelength a small amount (a few nanometers). Furthermore, both DBR and DFB lasers are difficult to construct at arbitrary wavelengths. Conventional external cavity diode lasers provide wavelength versatility and large continuous mode-hop free tuning ranges.
Conventional external cavity diode lasers, however, typically utilize various moving mechanical parts for electromechanically tuning the laser. For instance, a conventional tunable laser includes a cavity whose length may be mechanically adjusted so that the phase of the laser output signal can be electromechanically controlled for particular applications. Furthermore, intricate mechanical systems have been devised for controlling the frequency of the laser output. For instance, some conventional mechanically tunable lasers include a reflection grating which, depending upon the angle at which light strikes the grating, retro-reflects back only certain frequencies of light. In order to provide for large, continuous mode-hop free tuning, both the phase (total optical path length) and the frequency of the laser must be tuned in a synchronous and often complex mechanical fashion.
Conventionally, a frequency selective grating may be connected to one end of a pivot arm, which at its opposing end is fixed to a pivot point. The pivot arm and grating are then mechanically rotated in a highly precise and often complex manner so that only certain desired frequencies of light are reflected within the mechanically tunable laser cavity as desired. Furthermore, by appropriate choice of the pivot point, rotation of the grating about that pivot point results in the desired synchronous tuning of both the frequency and total optical path length of the laser. This can provide extended mode-hop free tuning. One such example of a mechanically tunable laser is described in U.S. Pat. No. 5,319,668 incorporated herein by reference.
These mechanical laser designs can utilize a wide array of laser diodes as the gain medium, thereby providing wavelength versatility. Furthermore, within the laser diodes gain profile, the mechanical external cavity diode laser can provide extended continuous tunability.
However, as recognized by the present inventors, mechanically tunable lasers have various limitations. First, it is difficult to make such mechanical devices compact. Second, such devices have limited lifetimes due to mechanical wear and failure issues. Third, mechanical devices are inherently vibration sensitive, which limits the type of environment in which they can be used. Finally, mechanical devices necessitate a level of design complexity including gears, bearings, and other mechanical components, which add to the cost, expense, and maintenance of such designs.
Accordingly, as recognized by the present inventors, what is needed is a tunable laser, with the desirable attributes of mechanically tuned external cavity lasers, which may be implemented without the need for any mechanically movable parts.
It is against this background that various embodiments of the present invention were developed.